comparison: huffman/bitreader.cpp
huffman/bitreader.cpp
- changeset 76
- 6de6d9a64ebd
- parent 8
- 8b697d30c49f
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| 75:5f8a03274d75 | 76:6de6d9a64ebd |
|---|---|
| 1 /* | |
| 2 * skulltag::BitReader class - Allows reading arbitrary bit lengths of data. | |
| 3 * Version 1 - Revsion 0 | |
| 4 * | |
| 5 * Copyright 2009 Timothy Landers | |
| 6 * email: code.vortexcortex@gmail.com | |
| 7 * | |
| 8 * Permission is hereby granted, free of charge, to any person obtaining a copy | |
| 9 * of this software and associated documentation files (the "Software"), to deal | |
| 10 * in the Software without restriction, including without limitation the rights | |
| 11 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | |
| 12 * copies of the Software, and to permit persons to whom the Software is | |
| 13 * furnished to do so, subject to the following conditions: | |
| 14 * | |
| 15 * The above copyright notice and this permission notice shall be included in | |
| 16 * all copies or substantial portions of the Software. | |
| 17 * | |
| 18 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | |
| 19 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | |
| 20 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | |
| 21 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | |
| 22 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | |
| 23 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | |
| 24 * THE SOFTWARE. | |
| 25 */ | |
| 26 | |
| 27 #include "bitreader.h" | |
| 28 /** Prevents naming convention problems via encapsulation. */ | |
| 29 namespace skulltag { | |
| 30 // BitReader class implementation | |
| 31 | |
| 32 int BitReader::intBitSize = 0; | |
| 33 int BitReader::intSize = 0; | |
| 34 int BitReader::mask[32] = {0}; | |
| 35 | |
| 36 /** Creates a new BitReader. */ | |
| 37 BitReader::BitReader(){ | |
| 38 init(); | |
| 39 } | |
| 40 | |
| 41 /** Creates a new BitReader. | |
| 42 * @param input Source of data that bits will be read from. | |
| 43 * @param max Maximum number of chars to read. */ | |
| 44 BitReader::BitReader( unsigned char const * input, int const &max ){ | |
| 45 inputBuffer( input, max ); | |
| 46 } | |
| 47 | |
| 48 /** Sets the input buffer that bytes will be read from. | |
| 49 * @param input Source of data that bits will be read from. | |
| 50 * @param max Maximum number of chars to input. | |
| 51 * @return true if successful, false if an error occurs. */ | |
| 52 bool BitReader::inputBuffer( unsigned char const * input, int const &max ){ | |
| 53 init(); // zero the vars. | |
| 54 currentByte = input; | |
| 55 if ( input == 0 ) return false; | |
| 56 if ( max < 1 ) return false; | |
| 57 bytesAvailable = max; | |
| 58 bitsAvailable = max << 3; | |
| 59 maximumBytes = max; | |
| 60 return true; | |
| 61 } | |
| 62 | |
| 63 /** Initializes this BitReader */ | |
| 64 void BitReader::init(){ | |
| 65 // initialize static variables if not initialized yet. | |
| 66 if ( intSize == 0 ){ | |
| 67 intSize = sizeof maximumBytes; | |
| 68 mask[0] = 0; | |
| 69 | |
| 70 // fill mask such that m = { 0, 1, 3, 7, 15, etc. } | |
| 71 for ( int i = 1; i < 32; i++ ) mask[i] = (mask[i-1] << 1) | 1; | |
| 72 | |
| 73 intBitSize = intSize << 3; | |
| 74 } | |
| 75 | |
| 76 // initialize member variables. | |
| 77 bitsAvailable = 0; | |
| 78 bytesAvailable = 0; | |
| 79 bufferBits = 0; | |
| 80 currentByte = 0; | |
| 81 maximumBytes = 0; | |
| 82 bitsUsed = 0; | |
| 83 } | |
| 84 | |
| 85 /** Fills the internal bit buffer. | |
| 86 * @return true if successful, false if an error occurs. */ | |
| 87 bool BitReader::fill(){ | |
| 88 if ( (currentByte == 0) || (bytesAvailable <= 0) ) return false; | |
| 89 | |
| 90 // while there's at least one octet free in the buffer, and one byte to read. | |
| 91 while ( (bitsUsed < (intBitSize - 8)) && (bytesAvailable > 0) ){ | |
| 92 | |
| 93 // put a byte into the bottom end of the buffer. | |
| 94 bufferBits |= (*currentByte & mask[8]) << (intBitSize - bitsUsed - 8); | |
| 95 | |
| 96 // Set variables to reflect the change. | |
| 97 currentByte++; | |
| 98 bytesAvailable--; | |
| 99 bitsUsed += 8; | |
| 100 } | |
| 101 return true; | |
| 102 } | |
| 103 | |
| 104 /** Fetches a specified number of bits and stores them in an int. | |
| 105 * @param bits destination of the retrieved bits. <br> | |
| 106 * The bits will be stored in the least significant portion of the int. | |
| 107 * @param count the number of bits to fetch. | |
| 108 * @return the number of bits read -- may not be equal to the amount requested. */ | |
| 109 int BitReader::get( int &bits, int const &count ){ | |
| 110 bits = 0; | |
| 111 // Requesting more bits than are available. | |
| 112 if ( count > bitsAvailable ) return 0; | |
| 113 if ( (count > bitsUsed) && (!fill()) ) return 0; | |
| 114 bits = (bufferBits >> (intBitSize - count)) & mask[count]; | |
| 115 // lesser of bits in buffer or requested bits. | |
| 116 int got = (bitsUsed < count) ? bitsUsed : count; | |
| 117 // get as many bits from the buffer as we can. | |
| 118 if ( got > 0 ){ | |
| 119 bufferBits <<= got; | |
| 120 bitsUsed -= got; | |
| 121 bitsAvailable -= got; | |
| 122 } | |
| 123 // if more bits are requested. | |
| 124 if ( count > got ){ | |
| 125 if (!fill()) { | |
| 126 bits = (bits >> (count - got)) & mask[count - got]; | |
| 127 return got; | |
| 128 } | |
| 129 got = count - got; | |
| 130 // avoid reading more bits than available. | |
| 131 if ( got <= bitsAvailable ){ | |
| 132 bits |= (bufferBits >> (intBitSize - got)) & mask[got]; | |
| 133 bufferBits <<= got; | |
| 134 bitsUsed -= got; | |
| 135 bitsAvailable -= got; | |
| 136 } | |
| 137 } | |
| 138 return count; | |
| 139 } | |
| 140 | |
| 141 /** @return Amount of bits that can be read from this BitReader. */ | |
| 142 int BitReader::availableBits(){ return bitsAvailable; } | |
| 143 | |
| 144 } // end namespace skulltag |
